This came up in a thread today, but I can't seem to post an answer there: related discussion:grass
Cf. also related discussion:jem. verpetzen -- to rat sb out
grass - (noun) ... 5 (slang) someone who betrays someone else, especially to the police. ... (verb) ... 3 (intrans, slang) (often 'grass on someone' or 'grass someone up') to inform on them, especially to the police.
grass - (n.) ... 4: (Brit., informal) a police informer. [ORIGIN: perhaps related to the 19th-cent. rhyming slang grasshopper 'copper.'] ...
(v.) ... 2: (intrans.) (Brit., informal) inform the police of criminal activity or plans: 'someone had grassed on the thieves.'
GRASS - to squeal or inform. "This word is derived from Cockney rhyming slang 'grasshopper,' meaning 'copper,' i.e., 'policeman.' 'Grass' sometimes appears as a noun, meaning both 'informer' or 'stool pigeon' and the 'act of informing' itself. It has shown up in the new form 'supergrass,' describing an I.R.A. member who turns 'queen's evidence' and names his former comrades." From "British English from A to Zed" by Norman Schur (FirstHarperPerennial edition, 1991). http://www.phrases.org.uk/bulletin_board/17/m...
grass - Noun. 1. An informer. Possibly from the rhyming slang grass in the park - 'nark', meaning informer. E.g."Don't tell John about this, he's a grass and I don't want to get into trouble." ... Verb. To inform (on), betray. http://www.peevish.co.uk/slang/g.htm